gistfile1.rb

#controllers/application.rb
...
class Hash
  def diff(other)
    keys = self.keys
    keys.each.select{|k| self[k] != other[k]}
  end
end

#config/init.rb
Merb::BootLoader.before_app_loads do
  ...
  require "lib/logger.rb"
  ...
end

Merb::BootLoader.after_app_loads do
  ...
  Misfit::Logger.start(['Clients','Branches','Centers','Loans'])
  ...
end


# lib/logger.rb
module Misfit
  module Logger
    def self.start(controllers)
      controllers.each do |c|
        puts "logging #{c}\n"
        cont = Kernel.const_get(c)
        cont.class_eval do
          include Misfit::ControllerFunctions
	end
	cont.add_filter(cont._before_filters, :get_object_state, {:only => ['update']})
        cont.add_filter(cont._after_filters, :_log, {:only => ['update','create']})
      end
    end
  end
end


module Misfit
  module ControllerFunctions

    def self.included(base)
      Merb.logger.info("#{base.to_s} included Controllerfunctions")
    end

    def self.extended(base)
      base.extend ClassMethods
      Merb.logger.info("#{base.to_s} extended Controllerfunctions")
    end


    def get_object_state
      model = self.class.to_s.singular
      object = eval"#{model}.get(params[:id])"
      @ributes = object.attributes
    end

    def _log
      f = File.open("log/#{self.class}.log","a")
      object = eval("@#{self.class.to_s.downcase.singular}")
      if object
        attributes = object.attributes
	diff = @ributes.diff(attributes)
        diff_string = diff.map{|k| "#{k} from #{@ributes[k]} to #{attributes[k]}" if k != :updated_at}.join("\t")
        log = "#{Time.now}\t#{session.user.login}\t#{diff_string}\n"
        f.write(log)
        f.close
      end
    end
  end
end